One way to provide power and data to an implanted electronic system such as a prosthetic stimulator is to transmit an RF signal via an inductive link. An inductive link basically has two resonant circuits: an external one and an internal one implanted in the patient user. The inductances of the two resonant circuits are realized, for example, as two spiral-shaped coils with typical outer diameters between 20 and 30 mm. When facing each other, the coils form a transformer which allows the transfer of RF-energy. Inductive links have been investigated with respect to optimizing power transfer efficiency and coupling misalignment tolerance. See, e.g., Galbraith D C, Soma M, and White R L, A Wide-Band Efficient Inductive Transdermal Power And Data Link With Coupling Insensitive Gain, IEEE Trans. Biomed. Eng. BME-34, pp. 265-275, April 1987; and Zierhofer C M and Hochmair E S, High-Efficiency Coupling-Insensitive Power And Data Transmission Via An Inductive Link, IEEE-Trans. Biomed. Eng. BME-37, pp. 716-723, July 1990; which are incorporated herein by reference.
In many applications, parallel-tuned receiver circuits are used because the RF-voltage across the resonant circuit can easily be converted to a dc voltage by rectification and smoothing. The dc voltage then is used as a power supply voltage for the electronic circuits within the implanted system. For example, FIG. 1 shows a parallel-tuned receiver resonant circuit of coil 101 and capacitor 102 where signal u2(t) is the induced RF-voltage. Rectifier diodes 103 and 104 in combination with filtering capacitors 105 and 106 convert the ac voltage u2(t) to a dc-like voltage Vdc. If the filtering capacitors 105 and 106 are sufficiently large, any ac components of Vdc can be neglected. Voltage Vdc is connected to voltage supply ports VCC and VSS of a subsequent electronic circuit 107 which implements the functionality of the implanted system, e.g., an implanted prosthetic stimulator.
Signal u2(t) is not only used as supply voltage generation for power, but it also contains digital information data. For example, for a cochlear implant, signal u2(t) provides information defining short biphasic pulses for the electrical stimulation of the acoustic nerve. In general, a bit decoding stage 108 is part of an implanted system that converts the RF-signal u2(t) to a base band bit sequence used for further processing.
For digital data transfer, at least two different distinguishable states of u2(t) are defined. For example, these two different states could be two different operating frequencies of u2(t), which are in the vicinity of the resonance frequency f2. Such a scheme is usually designated as Frequency-Shift-Keying (FSK). A practical example is described, e.g., in Galbraith above, where f2=20 MHz, and the two operating frequencies are 19 MHz and 21 MHz.
Another way to encode digital information in signal u2(t) is with Amplitude Shift Keying (ASK). In an ASK-scheme, the two distinguishable states of u2(t) can qualitatively be described by “RF-amplitude present” and “no RF-amplitude present”. These two (ideal) states can easily be detected by means of envelope detection. For example, in FIG. 1 decoding stage 108 would then include an envelope detector.
In FIG. 2, an equivalent circuit of an inductive link system is shown. The parallel-tuned receiver circuit includes receiver coil 201, capacitor 202, and resistor 208, where resistor 208 represents the ohmic losses due to the parasitic resistance of coil 201. Resonance frequency f2 and unloaded quality factor Q2,unloaded are defined as
                                          f            2                    =                      1                          2              ⁢              π              ⁢                                                                    L                    2                                    ⁢                                      C                    2                                                                                      ,        and        ,                            (        1        )                                          Q                      2            ,            unloaded                          =                              R            2                    ⁢                                                                      C                  2                                                  L                  2                                                      .                                              (        2        )            The power consumption of stage 107 in FIG. 1 is represented by an ohmic load 207. Rectifier diodes 103 and 104 are represented by simple equivalent circuits 203 and 204, which themselves are composed of ideal switches 2031 and 2041, and ohmic resistors 2032 and 2042. The states of the switches depend on voltage u2(t) and voltages VA and VB across capacitors 205 and 206, respectively. It is assumed that switch 2031 is closed if u2(t)>VA, and it is in its high impedance state for U2(t)≦VA. Similarly, switch 2041 is closed if u2(t)<−VB, and opened for u2(t)≧−VB.
Receiver coil 201 is inductively coupled to a transmitter coil 209, and the coupling strength is described by coupling coefficient k. Transmitter coil 209 together with capacitor 210 and resistor 211 form a series-tuned transmitter resonance circuit, where resistor 211 represents the parasitic resistance of coil 209. Resonance frequency f1 and unloaded quality factor Q1,unloaded are defined as:
                                          f            1                    =                      1                          2              ⁢              π              ⁢                                                                    L                    1                                    ⁢                                      C                    1                                                                                      ,        and        ,                            (        3        )                                          Q                      1            ,            unloaded                          =                                                                              L                  1                                                  C                  1                                                                    R              1                                .                                    (        4        )            The input of the transmitter circuit is driven by voltage source 212 which generates input voltage u1(t). For ASK, typically two modes of operation, i.e., states RF-ON and RF-OFF, are used. As depicted in FIG. 3, in state RF-ON, u1(t) is switched periodically between ground potential and a supply voltage VDD. Period T denotes the RF-period. During state RF-OFF, u1(t) is connected to ground potential.
FIG. 4 shows an example of voltage u1(t) for a sequence of bits using a self clocking bit format. Here, a logical “0” is encoded into a sequence RF-ON followed by RF-OFF, and vice versa, a logical “1” is encoded into a sequence RF-OFF followed by RF-ON.